Synchronization signal for synchronizing base stations

ABSTRACT

A synchronization signal used to synchronize base stations in a mobile radio telecommunication system having a first sequence followed by a second sequence, the first and second sequences being polyphase complementary sequences configured such that when the synchronization signal is correlated with a replica of the first sequence and a replica of the second sequence, and the correlation results are added exemplary synchronization results are obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.10/192,639, filed Jul. 11, 2002, which is a divisional application ofSer. No. 09/962,271, filed Sep. 26, 2001.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention concerns a signal for synchronizing base stationsin a mobile radio telecommunication system. More particularly, thepresent invention concerns a signal for synchronizing base stations fora telecommunication system of the time division duplex (TDD) type. Thetelecommunication system is for example the system for which a standardis at present being drawn up, normally referred to as 3GPP W-CDMA TDD.

FIG. 1 depicts a radio frame of such a telecommunication system. Itconsists of fifteen time slots, some of which, for example the slotsIT₀, IT₁, IT₂, IT₅, IT₆ and IT₈, are intended for conveying data (in thebroad sense of the term) in the downlink direction (base station tomobile terminal) whilst others, the slots IT₃, IT₄, IT₇, IT₉, IT₁₀,IT₁₁, IT₁₂, IT₁₃ and IT₁₄, are intended for conveying data in the uplinkdirection (mobile station to base station). During a transmission slot,the data (D) are transmitted in the form of a sequence of symbols. Theslot also includes a midamble (M) comprising pilot symbols enabling thechannel to be estimated, a power control word (TPC) and a guard period(GP′). In such a system, several mobile terminals or base stations cantransmit or receive data in the same time slot. The connections aredifferentiated by code division multiplexing (Code Division MultipleAccess=CDMA). The symbols transmitted by or for the different users arespectrally spread, approximately at a “chip” frequency 1/T_(c) where Tcis the elementary transmission period.

Because the same frequency can be used both in the uplink direction andin the downlink direction, it is essential to ensure synchronization ofthe base stations. This is because, if such were not the case, a firstmobile terminal transmitting at high power in an uplink channel couldinterfere with a second mobile channel, close to the first, receivingdata over a downlink channel. The synchronization constraint betweenadjacent base stations is around a few microseconds (approximately 5) inthe WCDMA TDD system.

To effect synchronization between base stations, several methods havebeen proposed in the state of the art. According to a first method, thesynchronization is achieved by virtue of GPS receivers equipping thebase stations. According to a second method, first of all, in an initialphase, for example during the phase of setting up the network or a newbase station, an approximate synchronization is carried out (of around afew tens of ms, that is to say a few tens of thousands of “chips”). Thisrough initial synchronization is provided by the network, or moreprecisely by the radio access controller (RNC) controlling severaladjacent base stations (also referred to as “B nodes”). A finesynchronization is then effected regularly by the radio interfacebetween adjacent base stations. The purpose of this fine synchronizationis notably to correct any difference in the sequencing clocks betweenadjacent base stations. To do this, certain time slots are reserved forthe transmission and reception of a synchronization signal. A time slotdedicated to synchronization comprises essentially a synchronizationsignal (Sync) and a guard period (GP). Synchronization is obtained, in amanner known per se, by correlation of the received sequence with asequence which is a replica of the one transmitted. The correlation iseffected on a time window with a length given by the margin of accuracyof the approximate synchronization. Thus, when a base station receives asynchronization signal and detects a correlation peak in this window, itcan synchronize its sequencing with that of the adjoining base stations.

The synchronization signal generally used is lengthy (a few thousands of“chips”) in order to obtain good accuracy of correlation for anacceptable power per symbol. The guard period must be greater than thepropagation time from a base station to an adjacent station so as toavoid, on reception, an encroachment of the synchronization signal on anadjacent time slot. The distance between two base stations being greaterthan the radius of a cell, the guard period (GP) is chosen so as to begreater than the normal guard period (GP′). The guard period (GP) mustalso take account of the difference between the frame clocks.

The synchronization signal is chosen so as to have good autocorrelationproperties, namely a very pronounced autocorrelation peak. Generally thesynchronization signals used are obtained from primitive polynomials onGF(2), a Galois field of cardinal 2. Such a sequence has a length Lwhich is an N^(th) power of 2 minus 1, that is to say L=2^(N)−1. This isthe case notably for so-called Gold sequences proposed in the reportTSGR1#15(00)0946 entitled “Sequences for the cell sync burst” of theWorking Group TSG-RAN of the ETSI for synchronizing adjacent basestations.

Gold sequences have good periodic autocorrelation properties (thecorrelation of a sequence consisting of the repetition of a Goldsequence with a replica of the sequence of the latter does not havesignificant secondary peaks). On the other hand, these sequencesunfortunately do not have such good aperiodic autocorrelation properties(correlation of an isolated Gold sequence with a replica). What is more,the correlator generally used operates in the time domain in the form ofa conventional adapted FIR filter having a complexity in terms of O(L)which can be very high. In addition, the choice of the lengths of suchsequences is reduced, since they can, as has been seen, take only values2^(N)−1 and a truncation would lead to a substantial loss ofautocorrelation properties.

SUMMARY OF THE INVENTION

One purpose of the present invention is to propose a signal forsynchronizing adjacent base stations by virtue of the transmission of acorrelation sequence having very good autocorrelation properties and awide choice of possible lengths, and this for a low degree of complexityof the correlator.

The present invention is defined by a signal for synchronizing basestations in a mobile radio telecommunication system in which a firstbase station transmits a synchronization signal having a first sequencefollowed by a second sequence, the first and second sequences beingobtained from polyphase complementary sequences, and at least one secondbase station effects the correlation of the synchronization signal witha replica of the first sequence and a replica of the second sequence,the correlation results then being added in order to providesynchronization information.

Advantageously, the first and second sequences are Golay complementarysequences.

According to a first embodiment, the synchronization signal comprisesguard times around the first and second sequences.

According to a second embodiment, the synchronization signal comprises aperiodic extension of the first sequence followed by a periodicextension of the second sequence.

According to a third embodiment, the first sequence is generated bymeans of a first Golay sequence and a first ancillary sequence bysuccessively multiplying the first Golay sequence by the bits of thefirst ancillary sequence.

Likewise, the second sequence can be generated by means of a secondGolay sequence, complementary to the first Golay sequence, and a secondancillary sequence by successively multiplying the second Golay sequenceby the bits of the second ancillary sequence.

Advantageously, the first ancillary sequence and the second ancillarysequence are Golay complementary sequences.

According to a variant embodiment, the correlation is effected by atrellis filtering.

BRIEF DESCRIPTION OF DRAWINGS

The characteristics of the invention mentioned above, as well as others,will emerge more clearly from a reading of the following descriptiongiven in relation to the accompanying figures, amongst which:

FIG. 1 depicts schematically a transmission frame of a transmissionsystem of the W-CDMA TDD type;

FIG. 2A depicts a first embodiment of the invention;

FIG. 2B depicts a second embodiment of the invention;

FIG. 2C depicts a third embodiment of the invention;

FIG. 3 depicts a correlator useful to the third embodiment of theinvention.

DETAILED DESCRIPTION

The general idea at the basis of the invention is to use, forsynchronizing adjacent base stations, a pair of complementary polyphasecodes and more particularly a pair of Golay complementary codes. In theremainder of the description, mention will be made not of polyphasecodes but of Golay codes. It is clear, however, that the inventionapplies to polyphase codes in general.

These complementary codes, known as such, have the remarkable propertythat the sum of their aperiodic autocorrelation functions is a Diracfunction. In other words, if a pair of such complementary codes isdenoted (A,B), this gives a□AA(m)+□BB(m)=□(m) where m is the time index,□ the Kronecker symbol, and □ the aperiodic autocorrelation function.

In addition, as described notably in the article by S. Z. Budisin,entitled “Efficient pulse compressor for Golay complementary sequences”,published in Electronics Letters, Vol. 27, N□3, pages 219-220 in January1991, the correlator can be produced by virtue of a trellis filterhaving a complexity in terms of O(logL) rather than in terms of O(L) asin a conventional adapted FIR filter. This trellis filter is alsoreferred to as an EGC filter, standing for Efficient Golay Correlator.An example of an embodiment of an EGC filter is given in the article byB. M. Popovic entitled “Efficient Golay Correlator”, published in IEEEElectronics Letters, Vol. 35, N□ 17, January 1999.

In addition, for a given authorized length, there are several possibleGolay sequences. This is because, Golay sequences being generated bygenerator codes, it can be shown that two distinct generator codes withthe same length generate Golay sequences which are also distinct andhave the same length. These sequences have good intercorrelationproperties (that is to say low intercorrelation values), enabling, forexample, groups of base stations to use distinct codes or again toeffect a synchronization of the base stations at different times oftheir sequencing.

A first embodiment of the invention is illustrated in FIG. 2A. Accordingto this embodiment, a synchronization signal consists of two Golaycomplementary sequences A and B multiplexed in time, each sequence beingpreceded and followed by a guard time, as described in the Frenchapplication FR-A-9916851 filed on 30/12/99 in the name of the applicant.This synchronization signal is transmitted by a base station and isreceived by an adjacent base station. On reception, the synchronizationsignal is correlated with a replica of the sequence A and a replica ofthe sequence B, and the result of correlation with the sequence A isdelayed so as to be aligned in time with the result of correlation withthe sequence B before they are added, the Dirac peak being obtained whenthe replicas of A and B are aligned with the corresponding sequences.The presence of the guard times GP₁, GP₂ and GP₃ ensures that, at thetime of correlation, the sequences A and B do not overlap thecorresponding complementary replicas, namely B and A respectively, in atime window centered on the time alignment position. Thus secondarycorrelation peaks can result from the intercorrelation between sequencesand complementary replicas are ejected out of this window. Moreprecisely, if GP₂=2.GP₃=2.GP₁=2.GP, the sum of the two correlationresults has an isolated Dirac peak in a window of width 2.GP around thetime alignment position. The correlations are advantageously effected byEGC correlators, as mentioned above.

A second embodiment of the invention is illustrated in FIG. 2B.According to this embodiment, a synchronization signal consists of twoGolay complementary sequences multiplexed in time, each sequence beingpreceded and followed by a periodic extension, as explained in theFrench application entitled “Channel estimation sequence and method ofestimating a transmission channel using such a sequence” filed in thename of the applicant. The periodic extension of a given sequence is atruncation of the periodic sequence obtained by repetition of thesequence. To do this, it suffices to concatenate with the sequence to beextended a prefix corresponding to the end and a suffix corresponding tothe start of the sequence. FIG. 2B indicates schematically theconcatenation of prefixes and suffixes for two Golay complementarysequences A and B. The synchronization signal itself consists of twosequences thus extended ext(A) and ext(B). The periodic extensionsproduce the same advantages as the guard times, namely the absence ofsecondary correlation peaks around the Dirac peak in a certain timewindow. More precisely, if the suffixes and prefixes are of identicalsize and equal to E, the sum of the correlation results will have anisolated Dirac peak in a window of width 2.E around the time alignmentposition. This will easily be understood if the case is considered wherethe synchronization signal comprises completely periodised sequences Aand B. The correlation with replicas of A and B then produces a seriesof Dirac peaks of period L. A periodic extension of size E amounts totruncating this series by a window of width 2.E around the timealignment peak. The advantage of this embodiment compared with theprevious one is not to cause abrupt variations in signal power betweenthe sequences A and B, at the transmitter amplifier. Such abruptvariations may generate high frequencies and intersymbol interferenceand consequently degrade the correlation results on reception.

A third embodiment of the invention is illustrated in FIG. 2C. Accordingto this embodiment, a composite sequence (10) is generated from a Golaycode sequence A or B and an ancillary sequence X (20), according to themode of constructing the hierarchical sequences. More precisely, thefirst bit of the ancillary sequence X (20) is multiplied successively byall the bits of the sequence A, and then the second bit of the secondsequence by all the bits of the sequence A, and so on, and he sequencesobtained are concatenated. Such a composite sequence will be noted belowA*X (30), A being the base sequence and X being the generator ancillarysequence (20). The Golay complementary sequences A and B can thus bemultiplied by ancillary sequences X, Y, identical or distinct, thelatter also being able themselves to be Golay sequences

Let A*X and B*X be composite sequences obtained from a pair A, B ofGolay complementary sequences, of length L, extended by prefixes andsuffixes of size E. A*X and B*X are multiplexed in time and separated byan interval W. The signal received is correlated with the sequence A onthe one hand and with the sequence B on the other hand.

The result of the first correlation is delayed by (L+2E)+W and is summedwith the result of the second correlation. The sum obtained is asequence R having a series of Dirac peaks of period L′=L+2E modulated bythe values x₀, x₁, . . . ,x_(K) where K is the length of the sequence X,each peak being surrounded by a window of width 2.E containing onlyzeros. The sequence R is then subjected to a filtering by means of alinear response filter:H(z)=x ₀ +x _(1.z) ^(−L′) + . . . +x _(k.z) ^(−K.L′).

The filtered sequence R includes a Dirac peak of height 2.K.L in themiddle of a zero window of width 2.E which makes it possible to detectit easily. In addition, the total sequence consisting of the sequencesA*X and B*X multiplexed in time is of total length 2.(L+2.E).K+W, whichoffers a wide choice of lengths of permitted sequences.

According to another variant embodiment, four composite sequences A*X,A*Y, B*X, B*Y are generated, where A, B form a first pair of Golaycomplementary sequences, extended or not, and X, Y form a second pair ofGolay complementary sequences serving as generator ancillary sequences.

The composite sequences are multiplexed in time and separated byintervals which will be assumed to be equal and of width W. Thesequences A and B are of length L′=L+2.E where L is the length of thebasic sequence and E the size of the extension, the sequences X, Y beingof length K. The total sequence length is therefore 4(L+2E)K+3W, whichoffers a wide choice of permitted sequence lengths.

The present variant takes advantage of the fact that there are L′ pairsof complementary sequences (X,Y) in the form of sub-sequences S_(m) andS′_(m) with S_(m)(n)=(A*X)_(n.L′+m) and S′_(m)(n)=(B*X)_(n.L+m), m=₀, .. . ,L′−1 obtained by decimation of the initial total sequence. Insteadof effecting a correlation with an EGC correlator, a “hierarchical”correlator is used, the first stage of the EGC function correlatormodified as depicted in FIG. 3.

It will be assumed that the pair of sequences X and Y has been generatedconventionally by an elementary sequence s₀, . . . ,s_(k−1), whereK=2^(k)−1, and delays D′₀, D′₁, . . . ,D′_(k−1) with D′_(i)=2^(Pi) where(P₀, P₁, . . . ,P_(k−1)) is a permutation on the set (0, 1, . . . ,k−1),recursively as follows:X ₀(i)=□(i); Y ₀(i)=□(i);X _(n)(i)=X _(n−1)(i)+s _(n−1) .X _(n−1)(i−D′i); Y _(n)(i)=Y _(n−1)(i)−s_(n−1) .Y _(n−1)(i−D′ _(i));Likewise, it will be assumed that the pair of sequences A, B wasgenerated by the elementary sequence t₀, . . . ,t_(l−1), where L=2¹−1,and delays D₀, D₁, . . . ,D_(k−1) with D_(i)=2^(Pi) where (P₀, P₁, . . .,P_(l−1)) is a permutation on the set (0,1, . . . ,l−1).

The first correlation stage effects a correlation with the pair ofsequences X, Y, but differs from a conventional EGC correlator in thatthe delays have been multiplied by a factor L′ in order to take accountof the scattering in the samples. The two correlation results are addedafter time alignment by a delay D_(XY), the delay D_(XY) separating thesequences A*X and A*Y, on the one hand, the sequences B*X and B*Y, onthe other hand. The second stage of the correlator effects thecorrelation with the pair of sequences A, B and is conventional per se.The correlation results are aligned in time by a delay D_(AB) and added,the delay D_(AB) corresponding to the difference in time between thesequences A*X and B*X on the one hand and the sequences A*Y and B*Y onthe other hand.

The correlator thus formed first of all effects a rough correlation witha step L′ and then a fine correlation to the sampling step. Itscomplexity is low since the number of operations performed is inO(log(K)+log(L)).

Although the example described above has only two sequence levels andtwo correlation levels, the invention can be extended in an immediatemanner to any number of levels of sequences and corresponding stages ofthe hierarchical correlator.

1. A synchronization signal used for synchronizing base stations in aradio telecommunication system, comprising: a first polyphase sequence;a second polyphase sequence following said first polyphase sequence,said second polyphase sequence complementary to said first polyphasesequence; and at least one of a periodic extension of said firstpolyphase sequence and a periodic extension of said second polyphasesequence connecting said second polyphase sequence to said firstpolyphase sequence.
 2. A synchronization signal used for synchronizingbase stations in a radio telecommunication system, comprising: a firstsequence consisting of a first Golay code and a periodic extension ofthe first Golay code; and a second sequence concatenated with said firstsequence and consisting of a second Golay code, complementary to thefirst Golay code, wherein said periodic extension of said first Golaycode is positioned between the first Golay code and the second Golaycode.
 3. A synchronization signal used for synchronizing base stationsin a radio telecommunication system, comprising: a first sequenceconsisting of a first Golay code; and a second sequence concatenatedwith said first sequence and consisting of a second Golay code,complementary to the first Golay code, and a periodic extension of thesecond Golay code, wherein said periodic extension of said second Golaycode is positioned between the first Golay code and the second Golaycode.
 4. A synchronization signal used for synchronizing base stationsin a radio telecommunication system, comprising: a first sequenceconsisting of a first Golay code, and a periodic extension of the firstGolay code; and a second sequence concatenated with said first sequenceand consisting of a second Golay code, complementary to the first Golaycode and a periodic extension of the second Golay code, wherein saidperiodic extension of said first Golay code and said periodic extensionof said second Golay code are positioned between the first Golay codeand the second Golay code.